The present invention relates to a method of bonding or coating a material to a substrate surface and to bonding together two substrate surfaces.
Despite a long history of adhesive and coating development, a need continues to exist for adhesives and coatings that provide increasingly higher bonding strengths under increasingly adverse conditions on an increasing variety of substrate surfaces.
A particular need exists for environmentally friendly aqueous or waterborne adhesive systems that avoid the use of volatile organic solvents. It has thus far been relatively difficult to develop aqueous adhesives that perform at a level equal to traditional solvent-based adhesives. One major problem associated with bonds formed from an aqueous adhesive is the relative susceptibility of the bonds to high temperature fluids and corrosive materials. Another need continues to exist for coatings or adhesives that deliver superior bonding capability at an inexpensive material cost. A further need exists for coatings or adhesives that can be applied with relatively few steps and minimal energy use. A few markets that are especially in need of a superior adhesive or coating are described below.
The manufacturing of articles, parts or assemblies that include an elastomer substrate surface bonded to another substrate surface (either another elastomer substrate or a non-elastomer substrate) typically involves placing the non-elastomer substrate in a mold, introducing a molten or liquid non-vulcanized (i.e., uncured) elastomer into the mold and then applying heat and pressure to simultaneously vulcanize the elastomer and bond it to the non-elastomer substrate. There are problems, however, with such vulcanization bonding. The molds often require a complicated design and interior profile, curing of the elastomer is slowed, there can be no incorporation of pre-compressed elastomer parts into the assembly, the assemblies undergo thermal stress, the product exiting the mold often has extra flashing that must be removed, any subsequent addition of more molded parts can significantly deteriorate the previously formed adhesive bond and there is limited process flexibility.
It would be advantageous under certain circumstances to bond the elastomer substrate surface to the other substrate surface after the elastomer substrate has been fully cured or vulcanized. This post-vulcanization bonding is sometimes referred to in the art as cold bonding. However, post-vulcanization bonding is one noticeable area in which adequate adhesive bonding is lacking, particularly when bonding to substrates made from different materials, especially metal or low surface energy materials. For example, cured ethylene-propylene-diene terpolymer rubber (“EPDM”) has a low surface energy that makes wetting difficult and it includes a relatively low amount of sites such as carbon-carbon double bonds that are useful in subsequent bonding. Adhesive bonding to post-vulcanized or cured elastomers has met with limited success. Cyanoacrylate adhesives are used for post-vulcanization bonding but these suffer from well known problems in more demanding industrial applications that are subjected to harsh environmental conditions. For example, cyanoacrylates suffer from poor heat resistance, solvent resistance and flexibility (see Handbook of Adhesives, edited by Skeist, I., pp. 473-476 (3d ed. 1990)). Other post-vulcanization adhesives are solvent-based and require high temperature and long curing times. Epoxy or urethane adhesives typically require elastomer surface pretreatment such as with oxidizing flames, oxidizing chemicals or electrical/plasma discharges in order to improve bonding. These pretreatment methods, however, are costly and time consuming.
The problems outlined above with current post-vulcanization adhesive bonding indicate that there is a long-felt need for an improved post-vulcanization adhesive bonding technique.
Another adhesive bonding area in which there continues to be a need is bonding to SANTOPRENE®, a commonly-used thermoplastic elastomer (“TPE”) commercially available from Advanced Elastomer Systems. Pre-cured and cured SANTOPRENE® TPE is particularly difficult to adhesively bond because it has a polyolefinic thermoplastic continuous matrix (similar to polyolefinic materials like polyethylene and polypropylene) that has an especially low surface energy of 28-30 dynes/cm according to U.S. Pat. No. 5,609,962. Bonding to more polar substrates such as metal and glass is practically impossible.
Tire retread bonding is another field in which there is a need for improvement. Tire tread replacement or retreading generally involves adhering a pre-cured or uncured retread stock to a cured tire carcass. The retread stock is placed circumferentially around the tire carcass with an uncured adhesive (known in the art as an adhesive cushion or cushion gum layer) disposed between the retread stock and the tire carcass. The resulting tire assembly is subjected to heat and pressure for a period of time to cure the adhesive cushion layer. If the retread stock is uncured, the process is occasionally referred to as “hot vulcanization” because the applied heat and pressure must be sufficiently great to vulcanize the retread stock. If the retread stock is pre-cured, the process is occasionally referred to as “cold vulcanization”. Application of the heat and pressure for the adhesive cure is typically accomplished by placing the tire assembly in a mold or autoclave which can require a significant amount of heat (generally from 80°-250° C. depending on whether the retread stock is pre-cured or uncured) for a significant time period (up to 300 minutes). Reduction or complete removal of this heat and pressure step would greatly contribute to a cost reduction for tire retreading.
It also would be advantageous to have a coating that can be applied without heat or extensive surface pretreatment, coat substrate materials that cannot currently be coated, has improved adhesion to the substrate surface and provide a waterborne coating for thermoplastic olefins that does not require heating.